Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling controls to EL2

From: Andrew Murray <andrew.murray@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kvm@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, Sudeep Holla <sudeep.holla@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling controls to EL2
Date: Thu, 9 Jan 2020 17:25:12 +0000	[thread overview]
Message-ID: <20200109172511.GA42593@e119886-lin.cambridge.arm.com> (raw)
In-Reply-To: <20191223121042.GC42593@e119886-lin.cambridge.arm.com>

On Mon, Dec 23, 2019 at 12:10:42PM +0000, Andrew Murray wrote:
> On Mon, Dec 23, 2019 at 12:05:12PM +0000, Marc Zyngier wrote:
> > On 2019-12-23 11:56, Andrew Murray wrote:
> > > On Sun, Dec 22, 2019 at 10:42:05AM +0000, Marc Zyngier wrote:
> > > > On Fri, 20 Dec 2019 14:30:18 +0000,
> > > > Andrew Murray <andrew.murray@arm.com> wrote:
> > > > >
> > > > > As we now save/restore the profiler state there is no need to trap
> > > > > accesses to the statistical profiling controls. Let's unset the
> > > > > _TPMS bit.
> > > > >
> > > > > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > > > > ---
> > > > >  arch/arm64/kvm/debug.c | 2 --
> > > > >  1 file changed, 2 deletions(-)
> > > > >
> > > > > diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> > > > > index 43487f035385..07ca783e7d9e 100644
> > > > > --- a/arch/arm64/kvm/debug.c
> > > > > +++ b/arch/arm64/kvm/debug.c
> > > > > @@ -88,7 +88,6 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu
> > > > *vcpu)
> > > > >   *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> > > > >   *  - Debug ROM Address (MDCR_EL2_TDRA)
> > > > >   *  - OS related registers (MDCR_EL2_TDOSA)
> > > > > - *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> > > > >   *
> > > > >   * Additionally, KVM only traps guest accesses to the debug
> > > > registers if
> > > > >   * the guest is not actively using them (see the
> > > > KVM_ARM64_DEBUG_DIRTY
> > > > > @@ -111,7 +110,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu
> > > > *vcpu)
> > > > >  	 */
> > > > >  	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) &
> > > > MDCR_EL2_HPMN_MASK;
> > > > >  	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> > > > > -				MDCR_EL2_TPMS |
> > > > 
> > > > No. This is an *optional* feature (the guest could not be presented
> > > > with the SPE feature, or the the support simply not be compiled in).
> > > > 
> > > > If the guest is not allowed to see the feature, for whichever
> > > > reason,
> > > > the traps *must* be enabled and handled.
> > > 
> > > I'll update this (and similar) to trap such registers when we don't
> > > support
> > > SPE in the guest.
> > > 
> > > My original concern in the cover letter was in how to prevent the guest
> > > from attempting to use these registers in the first place - I think the
> > > solution I was looking for is to trap-and-emulate ID_AA64DFR0_EL1 such
> > > that
> > > the PMSVer bits indicate that SPE is not emulated.
> > 
> > That, and active trapping of the SPE system registers resulting in injection
> > of an UNDEF into the offending guest.
> 
> Yes that's no problem.

The spec says that 'direct access to [these registers] are UNDEFINED' - is it
not more correct to handle this with trap_raz_wi than an undefined instruction?

Thanks,

Andrew Murray

> 
> Thanks,
> 
> Andrew Murray
> 
> > 
> > Thanks,
> > 
> >         M.
> > -- 
> > Jazz is not dead. It just smells funny...
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